(1) Field of the Invention
This invention relates to an apparatus having an automatic focusing (AF) function such as an AF camera, and more particularly to an automatic focusing device for use in a signal lens reflex camera, which is capable of accurately moving a focusing movable lens to a focused position.
(2) Description of the Prior Art
Recently, there have been available some lens-interchangeable cameras having an AF function.
FIG. 1 is a block diagram outlining the general construction of a interchangeable-lens camera of this type. More specifically, FIG. 1 mainly shows components of the camera which concerns the AF mechanism, not showing components which are generally provided for a camera and a lens.
In FIG. 1, reference numeral 11 designates a camera body; and 31, a photographical lens detachably mounted on the camera body 11. The camera body 11 and the lens 31 are mechanically connected to each other through a clutch 13 on the body side and a clutch 33 on the lens side, and are electrically connected to each other through an electrical contact group 15 on the body side and an electrical contact group 35 on the lens side.
The photographing lens 31 comprises: a lens system 39 including a focusing lens 37 which is movable along the optical axis; a drive force transmitting mechanism 41 for transmitting a drive force from a drive source (described later) to the lens 37 to move the latter 37 to a focusing position; and a lens ROM 43 for storing the aperture data of the photographing lens and the position data of the movable lens 37 and the like.
On the other hand, the camera body 11 comprises: an image pickup section 17 having, for instance, a CCD (charge-coupled device) sensor to receive a part of the light beam passed through the photographing lens 31 thereby to form the image of the object. The camera body 11 further comprises: a control section 19 having a variety of functions such as a function of calculating an amount of defocusing D indicating the amount of deviation from the focusing position and a direction of the deviation in response to the output signals of the image pickup section 17 and a function of determining the direction of movement of the lens for focusing the lens 37 on an object. The camera body 11 further comprises: a drive mechanism 25 including an electric motor 21 for driving the movable lens 37 in the photographing lens 31, and an encoder 23 for controlling the number of revolutions of the motor 21. The drive force of the drive mechanism 25 is transmitted through the clutch mechanism 13 and 33 and the drive force transmitting mechanism 41 to move the movable lens 37.
An amount of drive P moving the movable lens 37 to a focusing position can be determined in term of a number of rotation of the motor 21, that is, it can be set according to the pulse count number of the pulse signal from the encoder 23 which detects the number of revolutions of the motor 21. In the conventional AF camera, the amount of defocus D is calculated by the control section 19, and the pulse count value P corresponding to the amount of defocus D is calculated according to the following equation (1): EQU P=K.multidot.D (1)
where K is the lens movement conversion coefficient.
The coefficient K is predetermined so as to obtain a pulse count number P allowing the movable lens 37 to move to the position where the lens is positively focused on the object according to the amount of defocus D, and each photographing lens has its own coefficient K. The coefficient K is stored in the lens ROM 43 of the photographing lens. In the case where the photographing lens is a zoom lens, a plurality of coefficients are stored in the lens ROM.
The movable lens 37 is continuously moved until the number of pulses of the output signal from the encoder 23 reaches the pulse count value P calculated according to
equation (1).
In the case where it is required to focus the lens on an object with higher accuracy, the lens is driven intermittently; that is, the lens is moved small by small, and every time the lens is moved, a focusing pulse count value is newly obtained.
A method for obtaining an amount of defocus with respect to an object to be photographed will be described hereinafter.
First, the CCD sensor of the image pickup section 17 receives light from the object to provide defocusing data including an amount of defocusing and a direction of defocusing according to the correlation method. However, in the case where the present position of the movable lens is greatly apart from the focusing position for the object, sometimes it is impossible to obtain an amount of defocusing effective in calculation of the amount of drive to the focusing position, that is, sometimes the amount of defocusing cannot be obtained or the amount of defocusing includes an error. In this case, heretofore the following method is employed: The movable lens, after being moved for a distance in one direction at a constant speed, is stopped, and an amount of defocusing is obtained as described above. This operation is repeatedly carried out until the amount of defocusing thus obtained is determined effective. In the case where the lens is moved on in one direction, and no effective amount of defocus is obtained although the lens has reached the end point in the one direction, the lens is moved in the opposite direction, and the above-described series of operations are carried out until the amount of defocusing is determined effective.
However, when the movable lens is moved at a stretch according to the amount of drive of the movable lens obtained according the amount of defocus D, sometimes the resultant focal accuracy of the lens is lower than required.
This difficulty is significant when the amount of defocusing is large. That is, when the lens is greatly defocused from an object, the amount of defocusing obtained is often incorrect, including an error. Therefore, the amount of drive of the lens to the focusing position obtained from the amount of defocusing includes an error, as a result of which the lens is not satisfactorily focused on the object.
FIG. 2 is a graphical representation showing characteristic curves outlining the relation between an amount of defocus D and the number of pulses P counted by the encoder (hereinafter referred to as "a pulse count value P", when applicable) when the image having the amount of defocus is focused, with amounts of defocus D plotted on the horizontal axis and with the pulse count values P corresponding to the amounts of movement of the lens plotted on the vertical axis.
In FIG. 2, reference character I (solid line) designates an actual characteristic curve formed by plotting the numbers of pulses of a signal from the encoder when images having amounts of defocus are focused, respectively; and II (dotted line), a theoretical characteristic curve indicating the above-described expression (1).
As is apparent from FIG. 2, when it is required to determine the amount of movement of the focusing lens to the focusing position, the pulse count value can be accurately determined in the particular region 2d where the amount of defocus is small, whereas in the region where the amount of defocus is large, the pulse count value obtained includes a large error.
On the other hand, in the case where the movable lens is moved for a distance and stopped and then an effective amount of defocus is obtained by repeated trial and error, the light receiving operation of the CCD sensor is carried out after the movement of the lens has been accomplished, as a result of which it takes a relatively long period of time to obtain the amount of defocus, and accordingly a relatively long period of time is required for focusing the lens on the object. This is a serious difficulty for the camera, because it is essential for the camera to be able to achieve a photographing operating without missing an instantaneous shutter chance.